The present invention relates to converting pulverulent raw materials to a liquefied state as a first step in a melting process. The invention is generally applicable to processes that involve thermally converting a generally flowable, essentially solid state feed material to a molten fluid. The invention is particularly useful for liquefying glassy materials, including flat glass, container glass, fiber glass, sodium silicate glass, frits, cullet and the like, but is also applicable to liquefying ceramic materials, ores and other mineral materials.
U.S. Pat. No. 4,381,934 to Kunkle et al. teaches a method of converting particulate raw materials to a partially melted liquefied state on a support surface of batch material within a melting chamber. The initial process of liquefying batch material is isolated from the remainder of the melting process and is carried out in a manner uniquely suited to the needs of that particular step, thereby permitting the liquefaction step to be carried out with considerable economies in energy consumption and equipment size and costs. In the preferred embodiments of the Kunkle invention, thermal energy is supplied to a central cavity encircled by a layer of the raw material. Liquefied material is drained at the bottom end of the cavity and additional raw material is fed onto a top portion of the layer. The liquefaction takes place primarily in a surface transient layer while an underlying layer of the raw material remains substantially stable and unmelted. The material of the stable layer is chosen to be compatible with the throughput material, and therefore the stable layer acts as a non-contaminating, thermally insulating layer for the liquefying process. A steady state condition may be maintained in the liquefaction chamber by depositing fresh raw material onto the stable layer at essentially the same rate at which the material is melting, whereby a substantially constant thickness of the batch layer may be maintained to thermally protect the external structure of the melter. Because the partially melted material is in contact essentially only with material equivalent in composition, contaminating contact with refractories is substantially avoided.
While the protective lining of the stable layer of unmelted material remains essentially constant, there are minor fluctuations in the location of the interface between the stable layer and the transient layer. Irregularities in the interface generally correct themselves within a relatively short period of time with little or no disruption to the liquefaction process. However, occasionally a mass of the lining material may become dislodged and pass through the outlet before becoming entirely liquefied. It would be desirable to suppress these periodic deviations from steady-state operation.
In the Kunkle et al. type of liquefaction process, a steep slope on the lining relative to the axis of symmetry of the heated central cavity is advantageous in that it permits the surface of the unlined lid member to be maintained small relative to the lined surface area surrounding the cavity. In some embodiments, the slope of the lining can be increased by rotating the lining about the central cavity. In non-rotating embodiments, means for providing increased slope would be desirable. In rotating embodiments, such means would also be desirable to make the slope less dependent on the speed of rotation.
The Kunkle et al. process is not intended to complete the melting process, but rather to carry out only the initial liquefaction step. Completing the melting of undissolved particles and expulsion of gaseous inclusions from the liquefied material may be carried out in a separate stage downstream from the liquefaction chamber. For these subsequent steps it is often desirable to increase the temperature of the molten material above its liquefaction temperature. To assist in at least initiating such a temperature increase, it would be desirable in some cases to increase the residence time of the liquefied material within the liquefaction chamber so as to increase the temperature at which it leaves the liquefaction chamber.